1. Field of the Invention
The invention relates to a thin film transistor and a method of fabricating the same, and more particularly to a thin film transistor having a reverse-stagger structure and being capable of reducing off-leakage current, and a method of fabricating such a thin film transistor.
2. Description of the Related Art
There has been developed an active matrix type liquid crystal display device including a thin film transistor (TFT) as a switching device for a liquid crystal display. An active matrix type liquid crystal display device is designed to generally include an active matrix substrate having a gate wiring, a drain wiring, a thin film transistor, a pixel electrode and so on, an opposing substrate having a color filter, a black matrix layer and so on, and a liquid crystal layer sandwiched between the active matrix substrate and the opposing substrate. In an active matrix type liquid crystal display device, a direction in which liquid crystal molecules are oriented is rotated in accordance with a voltage applied either across electrodes arranged on both an active matrix substrate and an opposing substrate or between electrodes arranged in an active matrix substrate, to thereby control a light passing through a liquid crystal layer in each of pixels for displaying desired images on a display screen.
On the assumption that a semiconductor layer is formed above an active matrix substrate, a thin film transistor is grouped into a forward-stagger type one in which a gate electrode is formed above the semiconductor layer and source/drain electrodes are formed below the semiconductor layer, and a reverse-stagger type one in which a gate electrode is formed below the semiconductor layer and source/drain electrodes are formed above the semiconductor layer. A thin film transistor is generally designed to have a reverse-stagger structure.
FIG. 1 is a cross-sectional view of a conventional active matrix type liquid crystal display device having a reverse-stagger structure.
With reference to FIG. 1, in the conventional active matrix substrate, a gate electrode 2a is formed on a glass substrate 1, and a gate insulating film 3 is formed on the glass substrate 1, covering the gate electrode 2a therewith. On the gate insulating film 3 are formed an island-shaped amorphous silicon (hereinafter, referred to simply as xe2x80x9ca-Sixe2x80x9d) layer 4a which will make a semiconductor layer in a thin film transistor, and an n+ a-Si layer 4b containing much n-type impurity. The a-Si layer 4a and the n+ a-Si layer 4b are partially removed to define a channel 4d. A drain electrode 5a and a source electrode 5b are formed on the n+ a-Si layer 4b around the channel 4d. 
A passivation insulating film 7 is formed on the gate insulating film 3, covering the drain electrode 5a and the source electrode 5b therewith, in order to planarize a surface of the active matrix substrate.
The passivation insulating film 7 is partially removed above the source electrode 5b to define a contact 6. A pixel electrode 8 comprised of an electrically conductive transparent film such as a film composed of indium tin oxide (ITO) is formed in the contact 6 and each of pixels. An alignment film 9 is formed covering the pixel electrode 8 and the passivation insulating film 7 therewith.
Though not illustrated, red, green and blue color filters are formed on a glass substrate in each of pixels in the opposing substrate. An overcoating layer is formed on the color filters, and a transparent electrode composed of ITO is formed on the overcoating layer. Similarly to the active matrix substrate, an alignment film is formed on the transparent electrode in facing relation to the alignment film 9 of the active matrix substrate. The alignment films of both the active matrix substrate and opposing substrate are oriented into a predetermined direction.
The active matrix substrate and the opposing substrate are fixed to each other with spacers being sandwiched therebetween to define a gap therebetween. Liquid crystal is introduced into the gap.
In order to ensure desired switching characteristic in an active matrix type liquid crystal display device, it is generally important that an ON current, that is, a current running across source/drain electrodes when a gate is on is relatively high, and an OFF current, that is, a current running across source/drain electrodes when a gate is off is relatively small.
However, the conventional active matrix type liquid crystal display device having such a structure as mentioned above is accompanied with a problem that an off-leakage current is produced in a back-channel due to charging up in the spacers located above a thin film transistor and/or the alignment film 9, and causes malfunction in the thin film transistor with the result of display defect.
In order to prevent an off-leakage current to be produced in a back-channel, Japanese Patent No. 2621619 and Japanese Patent Publication No. 6-9246 have suggested an active matrix type liquid crystal display device in which an inactive layer is formed at a surface of an amorphous silicon layer which defines a channel.
Hereinbelow is explained the suggested active matrix type liquid crystal display device with reference to FIGS. 2A to 2C and 3.
FIGS. 2A to 2C are cross-sectional views of the active matrix substrate suggested in Japanese Patent No. 2621619, illustrating respective steps of a method of fabricating the same.
The active matrix substrate suggested in Japanese Patent No. 2621619 is fabricated as follows.
First, as illustrated in FIG. 2A, a metal film such as a chromium film is formed on an electrically insulating transparent substrate 19. Then, the metal film is patterned by photolithography and etching into a gate electrode 2a. 
Then, as illustrated in FIG. 2B, a gate insulating film 3 is formed on the electrically insulating transparent substrate 19, covering the gate electrode 2a therewith. Then, a semiconductor layer 20 is formed on the gate insulating film 3.
Then, as illustrated in FIG. 2C, the semiconductor layer 20 is exposed to hydrogen plasma 21 to thereby inactivate the semiconductor layer 20 at its surface.
Then, though not illustrated, a protection film is formed over the semiconductor layer 20, and there are formed source/drain electrodes which make electrical contact with the semiconductor layer 20 through contact holes formed around a channel. Then, a second protection film is formed covering a resultant therewith.
As mentioned above, in the method of fabricating an active matrix type liquid crystal display device disclosed in Japanese Patent No. 2621619, the semiconductor layer 20 is exposed to the hydrogen plasma 21 before the formation of a protection film for protecting a thin film transistor, to thereby increase a surface level at an interface between the semiconductor layer 20 and the protection layer. As a result, a back-channel in the thin film transistor is inactivated, and resultingly, a leakage current running through the back-channel while the thin film transistor is off can be reduced.
FIG. 3 is a cross-sectional view of an active matrix substrate suggested in the above-mentioned Japanese Patent Publication No. 6-9246.
In the suggested active matrix substrate, a gate electrode 2a composed of NiCr is formed on a glass substrate 1. A gate insulating film 3 is formed on the glass substrate 1, covering the gate electrode 2a therewith. An amorphous silicon layer 4a and an n+ amorphous silicon layer 4b are formed on the gate insulating film 3. A back-channel is formed by partially removing both the n+ amorphous silicon layer 4b and the amorphous silicon layer 4a by dry etching. The amorphous silicon layer 4a is formed at a surface thereof with a modified layer 22 containing oxygen, carbon and other element, by exposing the amorphous silicon layer 4a to plasma in a gas atmosphere in which at least one of nitrogen, oxygen, carbon and boron exists, in an apparatus in which a dry-etching process is carried out.
As mentioned above, in the above-mentioned Japanese Patent Publication No. 6-9246, after being dry-etched for forming a back-channel, the amorphous silicon layer is exposed to plasma in the above-mentioned gas atmosphere in an apparatus in which the semiconductor was dry-etched for forming a back-channel, to thereby form the modified layer 22, which is a stable layer, at a surface of the amorphous silicon layer 4a at the back-channel. This ensures reduction in an OFF current in a thin film transistor.
In accordance with the above-mentioned conventional methods of fabricating an active matrix substrate, it is possible to reduce an off-leakage current to some degree by inactivating a back-channel.
However, the first mentioned conventional method is accompanied with a problem that since the semiconductor layer 20 is exposed to atmosphere after being fabricated, the semiconductor layer 20 is contaminated at a surface thereof, resulting in probability in variance in potential at the back-channel.
The second mentioned conventional method overcomes the above-mentioned problem of contamination of the semiconductor layer, since the semiconductor layer is exposed to oxygen plasma in an apparatus in which dry etching is to be carried out.
However, both of the above-mentioned conventional active matrix substrates cannot be sufficiently inactivated by plasma, cannot effectively suppress generation of an off-leakage current, and cannot avoid display defect on a display screen.
In view of the above-mentioned problems in the conventional active matrix substrates, it is an object of the present invention to provide a thin film transistor and a method of fabricating the same both of which are capable of preventing an off-leakage current from being produced at a back-channel by spacers located above a thin film transistor and charging-up of an alignment film, and further preventing display defect.
In one aspect of the present invention, there is provided a method of fabricating a thin film transistor, comprising the steps of (a) forming a semiconductor layer above an electrically insulating substrate, (b) applying first plasma to the semiconductor layer through the use of a first gas, and (c) applying second plasma to the semiconductor layer through the use of a second gas.
It is preferable that the step (a) includes the step of forming the semiconductor layer composed of amorphous silicon or polysilicon.
It is preferable that each of the first and second gases contains at least one of oxygen (O), nitrogen (N), carbon (C), boron (B), hydrogen (H) and helium (He).
It is preferable that the first gas is comprised of oxygen, and the second gas is comprised of hydrogen and nitrogen.
It is preferable that the method further includes the step of heating the electrically insulating substrate at a predetermined temperature before applying the second plasma to the semiconductor layer.
The predetermined temperature is preferably in the rage of 250 degrees centigrade to 350 degrees centigrade both inclusive, and more preferably in the rage of 280 degrees centigrade to 320 degrees centigrade both inclusive.
It is preferable that the step of heating the electrically insulating substrate at the predetermined temperature is carried out for ten minutes or longer.
It is preferable that the second gas contains an element having an atomic number smaller than an atomic number of at least one of elements of the first gas.
There is further provided a method of fabricating a thin film transistor, comprising the steps of (a) forming a gate electrode on an electrically insulating substrate, (b) forming a gate insulating film on the electrically insulating substrate, covering the gate electrode therewith, (c) forming a semiconductor layer on the gate insulating film above the gate electrode, (d) forming source and drain electrodes both making electrical contact with the semiconductor layer, (e) patterning the semiconductor layer into a channel, (f) applying first plasma to the semiconductor layer through the use of a first gas, and (g) applying second plasma to the semiconductor layer through the use of a second gas, and (h) forming an electrically insulating film covering the semiconductor layer therewith.
The electrically insulating film in the step (h) is formed at power preferably in the range of 0.05 to 1.0 W/cm2 both inclusive, and more preferably in the range of 0.05 to 0.2 W/cm2 both inclusive.
There is still further provided a method of fabricating a thin film transistor, comprising the steps of (a) forming a semiconductor layer in a film-forming apparatus, (b) introducing a first gas into the film-forming apparatus to apply first plasma to the semiconductor layer, (c) introducing a second gas into the film-forming apparatus to apply second plasma to the semiconductor layer, (d) patterning the semiconductor layer into a channel, (e) forming a channel protection layer over the channel in the film-forming apparatus, and (f) forming an electrically insulating film covering the semiconductor layer therewith.
In the above-mentioned method in accordance with the present invention, there is no order in carrying out the step of applying first plasma to a semiconductor layer through the use of a first gas, and the step of applying second plasma to a semiconductor layer through the use of a second gas. The step of applying first plasma to a semiconductor layer through the use of a first gas may be carried out earlier than the step of applying second plasma to a semiconductor layer through the use of a second gas. As an alternative, the step of applying second plasma to a semiconductor layer through the use of a second gas may be carried out earlier than the step of applying first plasma to a semiconductor layer through the use of a first gas.
In another aspect of the present invention, there is provided a thin film transistor including a semiconductor layer including at least one element of a first gas and at least one element of a second gas, wherein the second gas contains an element having an atomic number smaller than an atomic number of at least one of elements of the first gas, and the element of the first gas is higher in concentration in the vicinity of the semiconductor layer than the element of the second gas, and the element of the second gas is higher in concentration than the element of the second gas in regions of the semiconductor layer deeper than the surface.
It is preferable that the element of the first gas is introduced into the semiconductor layer by applying first plasma to the semiconductor layer, and the element of the second gas is introduced into the semiconductor layer by applying second plasma to the semiconductor layer.
It is preferable that the first gas contains at least one of oxygen (O), nitrogen (N), carbon (C) and boron (B), and the second gas contains at least hydrogen (H).
It is preferable for the thin film transistor in accordance with the present invention to have such a back-gate characteristic that a drain current is equal to or smaller than 1xc3x9710xe2x88x9210 A when a drain voltage is equal to 10V, a front gate voltage is equal to xe2x88x9210V, and a back gate voltage is equal to 10V.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
The present invention makes it possible to suppress an off-leakage current at a back-channel by forming a stable inactivated layer, reducing a stress of an electrically insulating layer, and preventing the semiconductor layer from being damaged when the electrically insulating layer is formed.
The reason is as follows.
As will be explained later in the first embodiment, in accordance with the present invention, the first and second plasma steps are carried out after a semiconductor layer is etched for forming a channel, but before an electrically insulating film is formed. In the first plasma step, the semiconductor layer is exposed to oxygen plasma, and in the second plasma step, the semiconductor layer is exposed to hydrogen plasma. The second plasma step ensures that a region of the semiconductor layer into which oxygen atoms could not enter is inactivated.
In addition, the semiconductor layer is facilitated to be inactivated by heating a substrate at a predetermined temperature.
Furthermore, by forming an electrically insulating film on the inactive layer at power equal to or smaller than predetermined power, it would be possible to form a stable inactive layer, reduce a stress of the inactive layer, and prevent the inactive layer from being damaged while the electrically insulating film is being formed.
By carrying out the above-mentioned steps, it would be possible to form a stable inactive layer, and hence, suppress an off-leakage current at a back-channel.
As will be explained later in the second embodiment, an inactive layer is formed at a surface of a semiconductor layer by applying first plasma such as oxygen plasma to the semiconductor layer in an apparatus in which a channel protection layer is to be formed, and then, the inactive layer is further inactivated by heating a substrate at a predetermined temperature.
In addition, second plasma such as oxygen plasma is applied to the semiconductor layer to form an inactive layer in a region of the semiconductor layer into which oxygen atoms could not enter in the first plasma step.
As a result, it would be possible to form a stable inactive layer, and hence, suppress an off-leakage current at a back-channel.